https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=236888
Bug ID: 236888
Summary: ppp daemon limits mtu to 1492 for PPPoE
Product: Base System
Version: 11.2-RELEASE
Hardware: Any
OS: Any
Status: New
Severity: Affects Some People
Priority: ---
Component: bin
Assignee: b...@freebsd.org
Reporter: freebsd.68...@nospam.spacesurfer.com
The ppp daemon in FreeBSD 11.2 limits the maximum MTU to 1492 for the ppp/tun
device. Changing the MTU and MRU in the ppp.conf file to values above 1492
doesn't work. This is because the limit is hard coded in
/usr/src/usr.sbin/ppp/ether.c
My ADSL modem (vigor 130) supports an MTU above 1500 and if I change the limit
in ether.c to 1500, set my ethernet mtu to 1508 or more, then ppp will
negotiate a higher MTU/MRU for the link and it will work fine (I tried 1500
byte ping probes with the DF flag set and it works)
Unfortunately, you can't just change the ether.c value because it uses the
limit as the default and so my old modem (can only do 1492 or less) will not
work unless I set the MTU to 1492 in the ppp.conf file. It would probably be
best to leave the default at 1492 since this is the most common maximum MTU for
ADSL modems, but allow it to be overridden to whatever you want in the config
file.
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